Childhood and adolescent obesity is epidemic in the US, and the incidence is increasing. This is of concern because childhood obesity tracks into adulthood and is associated with higher rates of adult obesity and related co-morbidity. Adolescence is a critical period for the development of overweight, especially for girls. Also, on average, girls in the US have sub-optimal calcium intakes. Strong evidence supports the efficacy of a diet high in calcium, particularly in dairy food, in preventing overweight and facilitating weight loss in adults. Although observational studies suggest that calcium modulates weight in children, there are no published reports of randomized, controlled trials of the effect of dairy foods on weight/body fat (BF) in children or adolescents. The goal of the proposed research is to determine if increasing intake of dairy foods to recommended levels in adolescent females with habitually low calcium intake will decrease BF gain compared to similar females who continue their low calcium intake. We propose a 1-year intervention in 13- and 14-year old girls who are above the 50th percentile for body mass index (BMI) and are at least 1= years post- menarche to determine if we can prevent excessive BF gain. The hypothesis is: Post-menarcheal adolescent girls with habitually low calcium intake who consume dairy foods providing at least 1200 mg of calcium/day will have a smaller increase in percent BF during 1 year than post-menarcheal adolescent girls on a usual diet of 600 mg of calcium/day or less. We will recruit 321 healthy girls and conduct a 2-week run-in to assess their ability to tolerate a high dairy diet. We will enroll 274 eligible girls and randomly assign them, stratified by BMI- percentile (percentile), to a high dairy diet or their usual diet. Exclusion criteria include: total body bone mineral content (TBBMC) Z score <-2; pregnancy, lactose intolerance, early menarche, or any health problem. Girls > 85th percentile of BMI will be eligible only with approval of their personal physician. At 6-month intervals we will assess percent body fat, TBBMC, Tanner stage, height, weight, waist circumference, and abdominal girth. Dietary intake and physical activity will be assessed quarterly. The proposed study directly relates to the NINR Mission to prevent disease. Positive findings from this study will result in a safe, inexpensive nutritional method of preventing overweight and the related co-morbidity in adolescent girls. This may decrease their risk of adult obesity and will provide support for further studies in male adolescents and in younger children. PUBLIC HEALTH RELEVANCE: The purpose of this project is to determine if increasing intake of dairy foods to currently recommended levels (4 servings per day) in adolescent girls will decrease the risk of their becoming overweight. Positive findings from this study will form the basis for the future development of a safe, inexpensive, health-promoting method of preventing overweight in adolescent girls and provides the rationale for future studies in adolescent boys and in younger children. Such a dairy intervention would be congruent with currently recommended calcium and dairy intake.